Upgrading PC and RAM question

So i want to upgrade my pc but i dont know what to buy, specs below-

Processor: Intel Core i5 Quad Processor i5-750 ]
[RAM: Kingston 4GB DDR3 ]
[GPU: XFX GeForce GTX 260 869MB ]
550w chieftec psu

Oh and also, what if i have 1333mhz 4gb and buy 4gb more but with 1600mhz, will that decrease performance of the 1600mhz?, should i just buy another 4gig of 1333mhz?

Yea your rig is fine as is.

The only thing limiting performance in games will be the graphics card. As the others have said the RAM is really un-necessary at this point.

If you DID want to upgrade the graphics card, you would need to have enough cash to buy a 5850 or better to make it worth your while.

Thanks alot! I can run bad company 2 at max except shadows and hbao, its a little bit choppy sometimes, do i need a better gpu then?szafto

Pretty much. Video card is the only thing you could upgrade in your system that would make a significant difference.